1. Field of the Invention
This invention relates generally to the field of signal measurement, and in particular to accurate measurement and analysis of time interval related parameters of periodic signals such as electronic clock signals.
2. Discussion of the Related Art
Time interval related parameters of an electrical signal generally include period, pulse width, rise time, fall time, and frequency. These and other related features of the signal are measured from a system under test. Typically, the signal is periodic. Since the signal very often is not ideal, however, variations of the period, pulse width, frequency, and other parameters are indicative of the overall waveform stability of the signal. These variations are referred to as jitter. There are currently two types of equipment available for the analysis of signals to determine time interval related parameters and waveform stability.
The first type of equipment includes oscilloscopes. Generally, oscilloscopes operate by monitoring time windows of a waveform, and displaying portions of the monitored time windows. For example, an oscilloscope may be set to trigger on a particular voltage threshold of a waveform. As seen in FIG. 20, the oscilloscope monitors the waveform 200 during a first time window 202 and displays the beginning of the waveform 204 during the first time window 202, from the time that the set threshold was detected. There is a dead time 203, then the oscilloscope monitors the waveform during a second time window 206, and displays the part of the waveform 208 from the second time window 206 beginning at a time when the set threshold was detected. This sequence is repeated many times very quickly, for example for a third time window 209, so that a human eye does not detect that there is a dead time 203, 205 in between monitored time windows. Dead time may also be referred to as re-arm time, since it is often the time that it takes for the oscilloscope to re-arm the trigger.
A user views the display that includes parts of the waveform overlayed on one another, and interprets certain parameters. For example, as shown in FIG. 21, the time between two sequential rise times determines the period of the waveform 212. Since very often the waveform is not perfect, the display of the oscilloscope shows a number of lines that correspond to a number of instances of a rising edge of the waveform, shown at 214 in FIG. 21. The lines may be from a reconstruction of a digital waveform, or a composite of discrete points. The interval between the leftmost edge of the second rising edge and the rightmost edge of the second rising edge is referred to as jitter, shown as 216. In FIG. 21 the jitter appears as a time interval, although it actually represents a variation of the rise times among several cycles. Jitter may refer to any time variation of a waveform parameter, and is also called waveform stability. Other examples include period jitter, pulse width jitter and frequency jitter. Jitter may be instantaneous and vary significantly from cycle to cycle, or be long-term in that the jitter itself has behavioral characteristics that vary over time. Jitter is a critical parameter with respect to waveform analysis since it represents, among other things, the overall uniformity of the waveform. This measurement may be especially critical in very high speed processing applications.
There are many sources of error within an oscilloscope that adversely affect the accuracy of measurements such as jitter. As described above, the dead times 203, 205 may cause inaccuracies in jitter measurement, because the dead time 203, 205 represent many instances of cycles of the waveform that are simply not analyzed by an oscilloscope. Furthermore, the view observed on the display only represents a part 204 of the window 202 that was monitored. Therefore, the display is representative of only a fraction of all of the cycles of the waveform that occurred during the measurement and display time. Additionally, since the display is a composite of many rising and falling edges, it is not possible to determine with certainty from the display the characteristics of adjacent cycles, even though behavior of adjacent cycles can be extremely important.
Another source of error within oscilloscopes is referred to as trigger jitter or trigger interpolation jitter. In a digitizing oscilloscope, the waveform is first converted into a series of voltages. The series of voltages is then interpolated to provide a continuous voltage waveform to be displayed. However, since the waveform is being interpolated, the point at which the waveform crosses the trigger threshold must also be interpolated, so that the actual trigger point is the result of an interpolation. Accordingly, even the very best real time oscilloscopes have up to 36 picoseconds of error in a jitter measurement from trigger interpolation alone. Other less expensive equipment will have much more interpolation jitter error.
Oscilloscopes may be either real-time or equivalent-time. In real-time oscilloscopes, another source of error of time interval related parameters is sample clock jitter. The digital oscilloscopes perform analog to digital conversions that are triggered by a clock pulse. If the clock pulse used to trigger the analog to digital conversion is inaccurate, then the voltage converted at the time of the clock pulse will reflect this inaccuracy. Equivalent-time oscilloscopes can be more accurate, but not on a cycle to cycle basis, since the resulting display is a composite of several windows of measurement. Furthermore, an equivalent-time oscilloscope relies on several trigger events for one waveform representation, and may add error for each trigger event.
Another source of error for both real-time and equivalent-time oscilloscopes is front end analog noise, which is the noise inherent in the analog portions of the oscilloscope. Quantization error also adds to the total error, since an analog-to-digital converter will have a finite number of bits with which to represent the voltage of the waveform at a particular point. Error is also inherent in the creation of the signal display. This error, referred to as digital signal processing error, may be a result of imperfect interpolation, or other inaccuracies in the creation and interpretation of the signal display. As with most discrete time systems, aliasing is also a source of noise. Finally, there is error in reconstruction and interpolation from the quantized data, once it is to be displayed on the scope.
While trigger interpolation jitter alone may provide 36 or more picoseconds of noise, each of the sources of error described above combine to create a composite error. As a result of this composite error, the resulting accuracy of even the highest quality currently-available oscilloscopes is in the range of 50 picoseconds or greater.
A second type of equipment commonly used for time waveform analysis includes time interval analyzers. A time interval analyzer is essentially a very fast running counter, connected to a threshold and slope detector which monitors a waveform. A user selects a threshold and slope, and each time the selected threshold and slope is detected, the counter triggers and the time at which the slope is detected is recorded as the corresponding value of the counter at that time. The time interval analyzer performs analysis on the resulting series of trigger points.
There are several sources of error within a time interval analyzer. A large source of error is the front end analog circuitry, which is highly subject to noise. Another source of error is the counter itself, since the accuracy of the counter is highly dependent upon a clock which runs the counter. Additionally, a trigger event may occur between clock pulses to the counter, and the counter rounds off to either the preceding or following counter value. These sources of error often combine to provide approximately 600 picoseconds of jitter, even within the highest quality currently-available time interval analyzers.